a New Class of Sails. 203 



with carbonate of soda or carbonate of potash, according as 

 salts of sodium or potassium are desired. The neutralized 

 solution shows the presence of iron existing as a base, for 

 Prussian blue is precipitated on the addition of a prusside. 

 The neutral solution is now boiled, and it deposits generally 

 a green precipitate, though occasionally one of a brown co- 

 lour ; and the filtrate is found to be of a dark ruby-red, con- 

 taining only nitroprusside of the base employed and a nitrate. 

 The latter is separated by crystallization in the manner pointed 

 out under the respective salts. Nitroprusside of sodium, being 

 most easily prepared, is recommended as the product of the 

 process here given. 



Some practical difficulties may be mentioned so as to pre- 

 vent disappointment in the preparation. A carbonate of and 

 not the caustic alkali should be employed in the neutraliza- 

 tion. When the latter is used, the solution of nitroprusside 

 is apt to be mixed with ferrocyanide. When this takes place 

 an addition of acid serves to remove the impurity, as some of 

 the precipitated oxide of iron is dissolved, and forming prus- 

 sian blue with the ferrocyanide, removes it from the solution. 

 This impurity may also be removed by the addition of nitrate 

 of lead, which precipitates the prusside but not the nitroprus- 

 side ; or it may be taken away by the gradual addition of sul- 

 phate of iron, which removes the ferrocj'anide before precipi- 

 tating the nitroprusside. When the quantity last added pre- 

 cipitates the solution of a salmon colour, the impurity has 

 been removed*. 



Red prusside (ferridcyanide) of potassium may be used in 

 the preparation exactly as described for the yellow prusside. 



7. The following experiments were made in order to ascer- 

 tain approximatively how much nitroprusside was formed by 

 the process now described. 105*5 grs. crystallized ferrocy- 

 anide of potassium were digested with 1^ equiv. of nitric acid. 

 After digestion the liquid was neutralized with carbonate of 

 soda and boiled, the resulting green precipitate being collected 

 on a weighed filter. The filtrate was precipitated by a salt 

 of copper, and the nitroprusside of copper was collected and 

 weighed. 



It was found by various trials that perfectly uniform results 

 could not be obtained, the amount and even the composition 

 of the precipitatef on boiling varying with the conditions of 



• It is perhaps needless to remark, that when the ruby-red solution free 

 from prussides has been obtained by any of the processes above described, 

 it may be used at once for the precipitation of the insoluble nitroprussides. 



t The composition of the precipitates varies considerably. If on neu- 

 tralizing the acid solution an excess of alkali be added, the addition of an 



